UV-assisted nanoimprint lithography consists in duplicating patterns by pressing a mold, which is transparent at an usage wavelength situated in the UV range (i.e. a wavelength between 193 nm and 400 nm, preferably between 200 nm and 400 nm), in a UV photosensitive polymer film arranged in a substrate to be imprinted and applying UV radiation through said mold in order to photopolymerize the resin film.
The patterns reproduced in the polymer film are then etched in the substrate to be imprinted, underlying the polymer film.
It is called nanoimprinting because the duplicated patterns have a size (length, width and/or diameter) between a few nanometers and several hundred micrometers.
UV-assisted nanoimprinting is generally done using a technique called “step and repeat”: a mold having an area of several cm2 is put in contact with a resin film arranged on a substrate to be imprinted; the resin film is exposed to an usage wavelength situated in the UV range; the mold is then detached from the resin film and moved to another location of the substrate to be imprinted. These steps are repeated until the entire area of the substrate to be treated is imprinted.
The main drawback of this technique is that it is not possible to imprint two adjacent chips without leaving a separating distance between them. In the best of cases, this distance is several tens of micrometers, which constitutes a significant distance relative to the size of the chips and the size of the mold: there is therefore considerable lost space.
The main cause of this limitation comes from the fact that the mold usually made for carrying out the “step and repeat” technique is completely transparent to UV. As a result, during exposure to an usage wavelength situated in the UVs, the UVs modify the resin film present under the imprinted patterns, but also on the near periphery of the patterns (lateral diffusion).
This lateral diffusion of the light due to the transparency of the mold can be limited if the spaces situated between the patterns are covered by a light-absorbing layer. For example, the deposition of a layer of chrome on the face of a quartz mold comprising patterns will make it possible to locally eliminate the transparency of said mold. However, the deposition of said layer, done after the lithography and etching of the mold, is not simple to carry out and it is also difficult to very precisely define the boundary between the transparent zones and the absorbent zones of the mold. The presence of this light-absorbing layer also modifies the surface properties of the mold relative to the polymer film used during the nanoimprinting, which can cause new issues (appearance of flaws, wettability problem . . . ).
Another drawback of the UV-transparent molds is that they are generally rigid. Indeed, the molds generally used to perform UV-assisted nanoimprinting are made from quartz (quartz being transparent for wavelengths greater than 193 nm).
However, structuring using traditional lithography/etching methods of UV-transparent and rigid materials, and in particular quartz, becomes problematic when the patterns to be obtained have dimensions smaller than 100 nm.
Moreover, the more rigid a mold is, the more difficult it becomes to obtain homogenous contact (even contact) at all points between the mold and the resin film. It then becomes very difficult, or even impossible, to imprint patterns with satisfactory homogeneity. For example, a quartz mold has a rigidity such that the maximum area it is possible to imprint in a single step using such a mold is typically several cm2.
In light of the aforementioned drawbacks, the inventor's goal was to design a mold for nanoimprint lithography assisted by a determined wavelength in which the necessary distance between two successive chips can be minimized.
Another of the inventor's goals is for the mold also to be able to comprise patterns with a nanometric size and/or to have an area greater than several cm2.